The present invention relates to inversion means for converting electric power from direct (DC) to alternating (AC) form and, more particularly, relates to an improved control for such inversion means.
Electronic conversion of electric power from direct current (DC) to alternating current (AC) is generically referred to as inversion and the apparatus for performing such inversion is commonly referred to as an inverter. Electric power inversion can be practically accomplished by appropriately controlling the operations of a plurality of switching elements in alternative paths of load-current conduction between DC input (source) terminals and AC output (load) terminals of an inverter. The switching elements may comprise, for example, electric valves of the type having the ability to block current conduction until turned "on" by a suitable control or gate signal. One family of such valves is generally known by the names "controlled rectifier" or "thyristor," and the invention will be described using this family for switching elements. A detailed explanation of controlled rectifiers can be found in the General Electric SCR Manual, Fifth Edition, published 1972 by the General Electric Company, Semiconductor Products Department, Syracuse, New York.
There are many different circuit configurations and operating modes for inverters wherein controlled rectifiers are used as the main switching elements. By cyclically turning on and off the respective controlled rectifiers, an AC output voltage is derived from the DC power that is applied to the inverter input terminals. Any such inverter has to include suitable means for reliably turning off each controlled rectifier at the conclusion of its prescribed interval of load-current conduction and for assuring complete transfer of current from that "outgoing" controlled rectifier to the next-conducting controlled rectifier (the "incoming" rectifier), which transfer is called "commutation."
One well-known family of inverter circuits employs "impulse commutation" and a popular sub-family of impulse commutated inverter circuits is the "complementary impulse-commutated" inverter. A detailed explanation of complementary impulse-commutated inverters is set forth in chapter 7, pages 190-208 of The Principles of Inverter Circuits by B. D. Bedford and R. G. Hoft, published in 1964 by John Wiley and Sons, New York, New York.
In the complementary impulse-commutated inverter, current conduction in one load-current path is terminated by the onset of current conduction in an alternate path. In other words, the outgoing controlled rectifier is commutated through the action of turning on the incoming controlled rectifier. In order to effect commutation, this type of inverter requires that the applied load as seen by the inverter be inductive, i.e., the inverter requires a lagging power factor output current so that current will continue to flow in the load even though input power is interrupted. In order to assure a lagging power factor and to isolate the inverter from the driven load, a transformer is generally employed in the output or load terminals of this type inverter.
In the use of a complementary impulse commutated inverter, a problem arises if an attempt is made to start the inverter at operating voltage levels under conditions in which the output transformer may saturate before completion of the first half-cycle of inverter operation, a half-cycle being defined as the conduction period of one of the alternate current paths. This problem may occur, for example, if inverter operation has been interrupted for such a short interval that residual flux is present in the transformer core from a previous operation and the first controlled rectifier to be gated on results in a current flow which increases the flux level in the core. If the core then saturates, the commutating capacitor will not acquire sufficient charge to effect commutation of the first controlled rectifier when the second controlled rectifier is gated on and will result in a "shoot-through," or short-circuit, on the power lines causing, at the least, fuse burn out or destruction of the controlled rectifiers.
Prior art attempts to resolve this particular problem have involved costly additions to the power circuit. One method, described in U.S. Pat. No. 3,133,241 of David E. White, has been to include in the inverter circuit an additional high power controlled rectifier and associated logic circuitry to allow presetting of the transformer flux. Steering logic then assures that the controlled rectifier which will cause flux to be generated in a direction opposite to the present flux is gated on first. The above-described method suffers from the expense of an added power semiconductor and additional components which are subject to possible failure in a power circuit.
In addition to the above-described start-up problem, termination of the operation of prior art complementary impulse-commutated inverters has required that mechanical switches be provided in order to interrupt DC power to the inverter for at least a time period sufficiently long to allow the controlled rectifiers to cease conducting. The reason for the power removal is that the controlled rectifiers, once gated into conduction, will remain conducting, even though gate pulses are no longer applied, so long as current continues to flow in the controlled rectifiers. This additional DC power interrupting capability requires that the mechanical switches be capable of interrupting what may be relatively large currents thus making the switches subject to early failure due to arcing and burning and increasing the cost and difficulty of operating an inverter.
It is an object of the present invention to provide an improved control circuit and method of control for a complementary impulse commutated inverter.
It is another object of the present invention to provide an improved control circuit and method of control for a complementary impulse commutated inverter which circuit and method negate a need for additional power semiconductors.
It is still another object of the present invention to provide an improved control circuit and method of control for a complementary impulse commutated inverter, which circuit and method provide for inverter start-stop operation without the necessity of removing power from the inverter.